I very much like the idea of banker being able to impose realm-specific minimum/maximum trading prices. I could easily see a counter-argument that that should be left to the banker's influence so that there is yet another reason for internal conflict though. The same effect could be made, without imposing a hard limit, by allowing bankers to receive reports of all trades involving regions of the realm (or only regions whose granaries they have access to?). Then, a banker could announce such trading price policy, lords could either follow or not follow the banker's commands, but the banker is at least aware of their policy being broken, unlike now.
